The design of readout circuits for photo-detector array imaging systems is increasingly challenging due to larger format photo-detector arrays with smaller pixels, coupled with requirements for higher sensitivity and lower power dissipation. Traditional readout techniques involve photocurrent-to-voltage conversion at a pixel readout cell level using an integrating capacitor. Once the signals have been converted to voltage, time-multiplexing circuitry is used to transfer the discrete-time voltages to a single serial output. With smaller pixels, the size of the integration capacitor per readout cell is limited, thereby adversely affecting sampling noise. Additionally, larger format arrays require that the electronics multiplexing the discrete-time readout cell voltages operate at faster speeds for a given frame rate. For active imaging systems with moderately large photo-detector arrays, the bandwidth requirements may become prohibitive.
U.S. Pat. No. 6,618,125 to Stann entitled “Code-Multiplexed Read-out for Ladar Systems” (herein Stann), which is incorporated fully herein by reference, describes a readout scheme based on orthogonal modulation of photo-detector signals. In Stann, photocurrents are modulated with orthogonal carriers in the optical domain. Modulating photocurrents with the orthogonal carriers enables signals from an entire row to be read simultaneously, thereby improving the circuit bandwidth in comparison to conventional time-multiplexing schemes. The orthogonal modulation of photocurrents in the optical domain, however, is difficult and subject to losses in optical power. In addition, systems of this type still involve photocurrent-to-voltage conversion at the pixel readout cell level and, thus, have many of the same limitations at time-multiplexing schemes.
There is an ever present need for improved readout methods and apparatus for use with photo-detector arrays that are not subject to the above limitations. The present invention addresses this need among others.